1. Field of Invention
The current invention relates to magnetic resonance imaging systems, and more particularly to magnetic resonance imaging systems that provide reconstructed images of non-stationary organs.
2. Discussion of Related Art
Magnetic resonance imaging (MRI) is a reconstructive imaging technology that requires an exposure time, during which a subject's motion can pose a problem. For cardiac imaging, in particular, motion compensation is critical to obtaining high-quality, high-resolution diagnostic images.
Two different types of motion need to be compensated for during MR imaging of the heart, lung, liver, kidney, or other abdominal and thoracic organs. One is the respiratory motion and the other is the cardiac motion itself.
Respiratory motions in short scans may be removed by asking the subject under observation to engage in breath-holds for the duration of the scan. However, this limits the maximum duration of the scan, which in turn limits the achievable image quality as well as spatial and temporal resolutions of the image. However, high-resolution and high-quality images require longer scan times, making short scans non-feasible. Furthermore, certain patient populations, such as, for example, patients with heart failure, cardiac hypertrophy, or other cardiac conditions, can only hold their breath for a very short period of time or they cannot hold their breath at all. In these patients, breath-hold studies are very difficult, if not impossible. Most commercial scanners are equipped with respiratory bellows, which measure the general distention of chest cavity during breathing. However, these bellows are unreliable as they do not necessarily represent the true motion due to breathing and therefore their use is limited to simple patient monitoring and not for gating of imaging acquisition.
In free-breathing scans, conventional motion compensation techniques use respiratory navigator pulses that monitor a secondary site as an index of the respiratory motion. The secondary site may be, for example, the lung-liver interface on the right side of the body (also known as the right hemidiaphragm). Navigator pulses typically acquires a pencil beam 1D image that spans the lung-liver interface, producing a signal that can be monitored over time for each heart beat. The signal can be used to estimate motion in the foot-head direction. Navigator pulses can be used to accept or reject data based on the position of the organ of interest, or to correct acquired data to partially compensate for motion. Respiratory navigator pulses are generally effective, but can be difficult to prescribe on all patients. They are also difficult to implement on patients with variable respiratory patterns. Furthermore, and on a more fundamental level, these diaphragmatic navigator pulses attempt to estimate the motion of, for example, the heart, from the motion of another organ (the liver-lung interface), leading to errors. These navigator pulses assume the only relevant motion axis due to breathing is the foot-head direction and typically use fixed scaling to correct for cardiac motion, for example, for every 1 cm of liver motion, there may be an estimated 0.6 cm of motion at the base of the heart. These navigator pulses are also known to be limited in the sharpness of images because the resolution of the motion detectable with navigator pulses is maxed out for resolutions below approximately 0.75 mm. Finally, these respiratory navigator pulses are not in the same pulse sequence as the imaging pulse sequence for imaging the organ of interest. A separate pulse sequence is most useful for single-phase imaging, for example, diastolic imaging or coronary imaging, but is limited to non-steady-state acquisition and may not be useful in, for example, functional studies due to the interruption and disturbance of the steady state.
Typically, cardiac motion is compensated for by synchronizing the acquisition to an ECG waveform that is obtained by placing electrodes on the patient's chest. High-resolution imaging is achieved by combining MR imaging data acquired during corresponding portions of multiple cardiac cycles. This is a generally robust and widespread solution, and all MR system manufacturers have built-in ECG gating devices. However, ECG gating can fail for several reasons. First, at the high fields of the new clinical scanners (for example, those at 3.0 T or above) the ECG signal can become less reliable due to distortion of the ECG waveform by the magnetohydrodynamic effect. Second, and more importantly, ECG gating can lead to low quality images when the assumption of a periodic heartbeat breaks down. Cardiac MR images are normally acquired over several heartbeats, a process referred to as segmentation. All data is combined assuming that each heartbeat is identical to the previous one. For example, patients with cardiac arrhythmias (e.g. premature ventricular contractions) can have irregular heartbeats periodically, corrupting the acquired data and introducing artifacts into the MR images. It is known that a significant fraction of the population have high heart rate variability, that is, their heartbeat length varies a lot, even during a short period of time, which makes the assumption that all heartbeats are the same less true.
Recently, the concept of self navigation in which the raw magnetic resonance (MR) imaging data itself is used to identify, measure, and compensate for motion have emerged. Self-navigation may remove the need for external sensing devices to monitor both cardiac and respiratory motion. With self-navigation, the patient need not be disturbed to perform breath-holds (which is best for the sickest of patients who may be the ones most in need of an MR examination). The most basic self-navigation techniques attempt to estimate the underlying respiratory patterns, but more advanced and more recent techniques also extract the cardiac motion from the raw MR data itself.
Currently available self navigation techniques acquire low-spatial resolution data at high temporal resolution: every time image data is collected (also known as the repetition time of TR). Other navigator techniques acquire higher spatial resolution data every cardiac phase, sacrificing temporal resolution. All currently available methods have a net loss in efficiency, that is, either TR is extended to include the acquisition of the navigator data (reducing the fraction of time spent acquiring image data) or they extend the scan time, by taking complete TRs to acquire non-phase encoded data.
There are already several implementations of self-navigation techniques seen in the literature. A basic technique involves the acquisition of images in real-time (very fast) and using the images to determine the underlying motion of the heart (Kellman P, Chefd'hotel C, Lorenz C H, Mancini C, Arai A E, McVeigh E R. Fully automatic, retrospective enhancement of real-time acquired cardiac cine MR images using image-based navigators and respiratory motion-corrected averaging. Magn Reson Med 2008; 59(4):771-778; Pipe J G. Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging. Magn Reson Med 1999; 42(5):963-969; Leung A O, Paterson I, Thompson R B. Free-breathing cine MRI. Magn Reson Med 2008; 60(3):709-717). These techniques are generally limited by the amount of time it takes to acquire a complete image though they are able to resolve more complicated motion patterns.
Other self-navigation techniques that derive motion patterns from the raw data itself include techniques that use a multiple projections for images and techniques that use a single projection acquired repeatedly. For the latter category, the projection can be acquired sporadically (e.g., every cardiac phase with relatively low temporal resolution) every time imaging data is acquired. This approach incurs an extra cost of time in the TR (e.g. a free-induction-decay, or FID, signal is read-out at the beginning of the TR or an extra echo at the end of the TR) even when performed with very low resolution (Leung A O, Paterson I, Thompson R B. Free-breathing cine MRI. Magn Reson Med 2008; 60(3):709-717; White R D, Paschal C B, Clampitt M E, Spraggins T A, Lenz G W. Electrocardiograph-independent, “wireless” cardiovascular cine MR imaging. J Magn Reson Imaging 1991; 1(3):347-355; Wyatt C A, N; Kraft. R. Spherical Navigator Registration Using Harmonic Analysis for Prospective Motion Correction. 2005. Springer-Verlag. p 738-749; Fu Z W, Wang Y, Grimm R C, Rossman P J, Felmlee J P, Riederer S J, Ehman R L. Orbital navigator echoes for motion measurements in magnetic resonance imaging. Magn Reson Med 1995; 34(5):746-753; Welch E B, Manduca A, Grimm R C, Ward H A, Jack C R, Jr. Spherical navigator echoes for full 3D rigid body motion measurement in MRI. Magn Reson Med 2002; 47(1):32-41; Hiba B, Richard N, Janier M, Croisille P. Cardiac and respiratory double self-gated cine MRI in the mouse at 7 T. Magn Reson Med 2006; 55(3):506-513; Larson A C, White R D, Laub G, McVeigh E R, Li D, Simonetti O P. Self-gated cardiac cine MRI. Magn Reson Med 2004; 51(1):93-102; Crowe M E, Larson A C, Zhang Q, Carr J, White R D, Li D, Simonetti O P. Automated rectilinear self-gated cardiac cine imaging. Magn Reson Med 2004; 52(4):782-788; Larson A C, Kellman P, Arai A, Hirsch G A, McVeigh E, Li D, Simonetti O P. Preliminary investigation of respiratory self-gating for free-breathing segmented cine MRI. Magn Reson Med 2005; 53(1):159-168; Hiba B, Richard N, Thibault H, Janier M. Cardiac and respiratory self-gated cine MRI in the mouse: comparison between radial and rectilinear techniques at 7 T. Magn Reson Med 2007; 58(4):745-753; Lai P, Larson A C, Bi X, Jerecic R, Li D. A dual-projection respiratory self-gating technique for whole-heart coronary MRA. J Magn Reson Imaging 2008; 28(3):612-620; Buehrer M, Curcic J, Boesiger P, Kozerke S. Prospective self-gating for simultaneous compensation of cardiac and respiratory motion. Magn Reson Med 2008; 60(3):683-690; Lai P, Larson A C, Park J, Can J C, Li D. Respiratory self-gated four-dimensional coronary MR angiography: a feasibility study. Magn Reson Med 2008; 59(6):1378-1385; Stehning C, Bornert P, Nehrke K, Eggers H, Stuber M. Free-breathing whole-heart coronary MRA with 3D radial SSFP and self-navigated image reconstruction. Magn Reson Med 2005; 54(2):476-480).
Thus, there is a need for an improved magnetic resonance imaging system for use with non-stationary organs.